How do I begin my personal statement?

It's always important to plan and I'd suggest you mind map your ideas. Write the subject you want to study in the middle of the bubble and write around it what aspects of the course/subject you enjoy or find interesting. Around each of those write why it is you enjoy them and then try and fit any additional reading or extra curricular activities you do around those things. Once you get into the flow, it'll become less daunting and easier!